The Nender is a dimension where you fight the nether dragon and defeat it. It is composed mostly of glowstone and netherrbrick.
Nether Crystals are like End Crystals, except they also act as blaze and ghast spawners.
The Nender is just an opening in an infinate world of Netherbrick and glowstone, and don't worry you spawn in the opening.
The Nether Dragon looks like a dark red Ender Dragon and has 150 hearts of health.